Physical Sciences Division Research Highlights

New Methods Detect Radionuclides in Groundwater

Collaboration key on technetium-99 and strontium-90 sensing

A strontium-90 groundwater monitoring platform deployed on the north Hanford Site. The instrument draws water from a shallow well near the shore of the Columbia River.

Using this sensor, researchers can measure the amount of strontium in groundwater. The strontium (yellow) is captured on the beads. The strontium decays to yttrium (magenta) which is not attached to the beads. The yttrium is washed to a detector where it is measured. Simple calculations are then used to determine the amount of strontium that produced the measured level of yttrium. » » Flash Video

Results: Bringing together science and engineering, a team at Pacific Northwest National Laboratory and Burge Environmental figured out how to measure the amount of radioactive technetium-99 and strontium-90 in the groundwater under some weapons sites. These systems acquire data in close to real time and transmit it at as needed to monitor and remediate these radioactive troublemakers. The systems collect data without the expense and time of field sampling and laboratory analysis.

Scientists at Pacific Northwest National Laboratory developed the sensitive approaches to measure the radioactive isotopes. The measurement approaches are simple and robust. Engineers at Burge Environmental, Inc., designed and built the solar-powered systems or platforms that can function, unattended, in the field.

"In this manner, we can bring groundwater analysis out of the laboratory and to the field." said Matt O'Hara, project lead at PNNL.

Why it matters: At the Hanford Site in southeastern Washington State and other nuclear sites around the world, portions of the groundwater contain radionuclides. This contaminated groundwater can flow, depending on the site's geology, into nearby rivers and other water sources. The people managing these sites need to continuously monitor the contaminants in the groundwater. In some cases, groundwater remediation is required.

"We have developed simple and robust methods for detecting radionuclides in groundwater," said O'Hara.

Methods: Measuring the concentration of strontium-90, technetium-99, and other radionuclides in groundwater presents challenges. To be useful, the sensor must detect minute quantities in large sample volumes. Also, the water samples are complex, containing nonradioactive salts and potentially interfering radionuclides. So, these sensors must be selective to detect only the desired analyte.

Troublesome Technetium-99. Underneath the Hanford Site, a highly mobile form of technetium-99 called pertechnetate moves through the groundwater. Combined with its slow decay rate, taking 211,000 years to decay by half, technetium-99 is a concern.

The new radiochemical sensor, in its simplest form, is a small column filled with two types of beads. The first material, anion exchange resin, captures the pertechnetate from the groundwater. The second material, scintillating beads, enables the concentration of the radionuclide to be measured.

As groundwater is pumped through the column, the anion exchange beads selectively grab pertechnetate. When the technetium decays it emits a beta particle. These particles strike the scintillating beads, which in turn give off light in proportion to the number of beta particles that impinge upon them. By measuring the amount of light emitted from the scintillating particles, researchers can determine how much of the isotope is in the sample. The sensor can quantify the technetium-99 levels down to at least 10 times lower than the drinking water standard, 33 Bq/L.

Sensing Strontium-90. In the sensor, strontium is captured in a column packed with tiny, highly selective beads or sorbent. After all the groundwater sample has flowed through the column, the column is washed, removing any decay products and leaving pure strontium on the sorbent.

Next, a timer begins.

While the timer counts down, some of the strontium-90 stuck to the beads decays to yttrium-90. The yttrium is not bound to the beads. When time is up, the yttrium is transported to a detector, while the strontium remains on the column. The amount of yttrium-90 is measured via the radiation that it emits.

Using a simple formula, the researchers calculate the amount of strontium that produced the measured level of yttrium. The sensor can measure 0.057 Bq/L of strontium in a liter of sample, well below the drinking water limit of 0.30 Bq/L. At the end of the analysis, the strontium left on the column is stripped and sent to waste. Now the system is prepared for a subsequent analysis.

"People have used the strontium/yttrium decay scheme before," said chemist Jay Grate from PNNL. "But, nobody has put it into an automated monitor like this."

The system is designed mostly for low levels of contamination, but the team wanted the sensors to be capable of measuring high levels of strontium-90 as well. So the team developed a high-level mode that measures yttrium-90, which occurs together with Sr-90, directly from the groundwater sample. This enables a more rapid analysis and may help study short-term plume phenomena such as those caused by changing levels in the river.

What's next? The team is fitting these two analytical methods into the Universal Sensor Platform. The platform is capable of remote, off-the-grid operation, and each unit is capable of sampling from up to five different wells or other groundwater sources. The first prototype was recently deployed to the 100N Area of the Hanford Site (see photo), where it will monitor the behavior of strontium-90 in a shallow groundwater well adjacent to the Columbia River. Additional platforms are planned for deployment on the Hanford Site later in 2009.